High voltage in a Blackface Bandmaster

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guitarjesus
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High voltage in a Blackface Bandmaster

Post by guitarjesus » Thu Jan 18, 2018 4:08 pm

Years ago I was a dumb kid, couldn't afford a Marshall and I let con-man Dan Torres take my BFBM and replace the Ajax caps with Orange Drops, put a JTM 50 watt output transformer and EL34s in it. Yes, I was dumb, BUT I was 15.

NOW, I'm putting my amp back to stock and slowly getting parts back.

Poking around in the amp with a volt meter one of the measurements is way off. On the vibrato tube the schematic is calling for 440v going into the 220k plate resistor and 280v coming out. I'm getting 435, and the resistor os testing good at 227k. What should I be looking for to remedy this? An extra 150v going to a 12ax7 can't be good.

danman
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Re: High voltage in a Blackface Bandmaster

Post by danman » Fri Jan 19, 2018 6:13 pm

That almost seems as if the tube, or at least that half of the tube isn't conducting. First thing would be to try a new tube. If that doesn't help, check that the cathode resistor for that side of the tube has not gone open or lost it's ground connection. You may need to unsolder one end of the resistor from the board to check if it's reading it's proper value. Another possibility maybe a bad connection in the tube socket. Be sure it fits snugly and try reinserting it a couple times.

guitarjesus
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Re: High voltage in a Blackface Bandmaster

Post by guitarjesus » Tue Jan 23, 2018 9:26 pm

Found the problem. Apparently until the tremolo footswitch jack grounds the optoisolator, with the ground lifted the voltage cant drop across the resistor with nowhere for the voltage to go. Once the circuit is complete, the voltage fluctuates as the vibrato does.

Now I know, all is well.

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